This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Field Of The Invention
The present disclosure relates to the field of well completions. More specifically, the present invention relates to the isolation of formations in connection with wellbores that have been completed using gravel-packing. The application also relates to methods for isolating subsurface intervals along an open-hole wellbore.
Discussion Of Technology
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling to a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is typically conducted in order to fill or “squeeze” the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of formations and aquifers behind the casing.
It is common to place several strings of casing having progressively smaller outer diameters into the wellbore. The process of drilling and then cementing progressively smaller strings of casing is repeated several times until the well has reached total depth. The final string of casing, referred to as a production casing, is cemented in place and perforated. In some instances, the final string of casing is a liner, that is, a string of casing that is not tied back to the surface.
As part of the completion process, a wellhead is installed at the surface. The wellhead controls the flow of production fluids to the surface, or the injection of fluids into the wellbore. Fluid gathering and processing equipment such as pipes, valves and separators are also provided. Production operations may then commence.
It is sometimes desirable to leave the bottom portion of a wellbore open. In this instance, production casing is not set in the wellbore, but the well is instead completed as an “open-hole wellbore.” In open-hole completions, a production casing is not extended through the producing zones and perforated; rather, the producing zones are left uncased, or “open.” A production string or “tubing” is then positioned inside the wellbore extending down below the last string of casing and across a subsurface formation.
There are certain advantages to open-hole completions versus cased-hole completions. First, because open-hole completions have no perforation tunnels, formation fluids can converge on the wellbore longitudinally and radially 360 degrees. This has the benefit of eliminating the additional pressure drop associated with converging radial flow and then linear flow through particle-filled perforation tunnels. The reduced pressure drop associated with an open-hole completion virtually guarantees that it will be more productive than an unstimulated, cased hole in the same formation.
Second, open-hole techniques are oftentimes less expensive than cased hole completions. For example, open-hole completions generally do not require cementing, perforating, and post-perforation clean-up operations. However, open-hole completions carry their own set of problems.
A common problem in open-hole completions is the immediate exposure of the wellbore to the surrounding formation. If the formation is unconsolidated or heavily sandy, the flow of production fluids into the wellbore may carry with it formation particles, e.g., sand and fines. Such particles can be erosive to production equipment downhole and to pipes, valves and separation equipment at the surface.
To control the invasion of sand and other particles, sand control devices may be employed. Sand control devices are usually installed downhole across formations to retain solid materials larger than a certain diameter while allowing fluids to be produced. A sand control device typically includes an elongated tubular body, known as a base pipe, having numerous pre-drilled holes, or slotted openings. The base pipe is then typically wrapped with a filtration medium such as a screen or wire mesh.
To augment sand control devices, particularly in open-hole completions, it is common to install a gravel pack. Gravel packing a well involves placing gravel or other particulate matter around the sand control device after the sand control device is hung or otherwise placed in the wellbore. To install a gravel pack, a particulate material is delivered downhole by means of a carrier fluid. The carrier fluid with the gravel together form a gravel slurry. As the carrier fluid is pumped downhole, it moves into the formation, leaving a circumferential packing of gravel around the screen. The gravel not only aids in particle filtration but also helps maintain formation integrity by supporting the surrounding formation.
In an open-hole gravel pack completion, the gravel is positioned between a sand screen that surrounds a perforated base pipe and a surrounding wall of the wellbore. During production, formation fluids flow from the subterranean formation, through the gravel, through the screen, and into the pre-slotted base pipe. The base pipe thus serves as a part of the production string.
It is known to install multiple sand screens across multiple producing intervals. This arrangement works fine as long as each interval is producing hydrocarbon fluids, particularly oil. However, in some instances an interval may begin to produce unwanted volumes of water. This may be due to a fingering of water from an injection well. Alternatively, such may be due to a rise in the water level subsurface, such as in connection with a water-drive reservoir or due to the presence of native water zones, coning (rise of near-well hydrocarbon-water contact), high permeability streaks, or natural fractures. Depending on the mechanism or cause of the water production, the water may be produced at different locations and times during a well's lifetime. In any of these instances, it is desirable to be able to isolate that interval to shut off or at least inhibit the flow of water into the wellbore.
An interval may alternatively begin to produce an unwanted volume of gas. This may be due to a breakthrough of gas at an interval in connection with an enhanced oil recovery operation. Alternatively, a gas cap above an oil reservoir may expand and break through, causing gas production with oil. The gas breakthrough reduces gas cap drive and suppresses oil production. Once again, it is desirable to be able to isolate that interval so as to shut off or at least inhibit the flow of gas into the wellbore.
In the case of open hole gravel pack completions, the isolation of zones or intervals is problematic. Specialized bypass packers are being developed that allow for a certain amount of annular isolation. During the completion phase, packer locations are chosen so that potential troublesome regions of the well such as higher permeability zones can be isolated. This also assists later during workover operations. While the use of specialized packers is increasing, numerous older wells exist that do not have zonal isolation capabilities. As these wells mature, they can experience an unwanted inflow of water or gas as described above.
Various workover approaches have been employed for zonal isolation in older completions. For example, cement squeezing may take place along an intermediate zone. The cement is delivered using coiled tubing or a wireline tool. The hope is to plug a section of sand screen. Alternatively, cement may be dumped into the bottom of the well to close off a lower zone. However, the use of cement may be expensive and has the potential of plugging productive subsurface intervals along with encroachment intervals.
A foam compound may also be injected into an intermediate zone using coiled tubing. The foam compound serves to plug a selected interval. However, foam compounds generally do not hold for long since the water degrades the foam with time, or the water finds new paths around the plugged area.
Therefore, a need exists for an apparatus that isolates fluid flow along a selected zone in an open-hole, gravel pack completion. In addition, a need exists for a method of isolating fluid inflow along a selected subsurface interval in an open-hole wellbore in a cost-effective manner.